Method and apparatus for anaerobic digestion of biomasses and generation of biogas

ABSTRACT

Method and apparatus for an anaerobic digestion of biomasses with generation of biogas and sludge, wherein a hydrolyzation phase and a following fermentation of the biomass are included, wherein a hydrolyzation phase ( 5 ) and at least two following fermentation phases ( 7,8,9 ) are included and wherein the hydrolyzation ( 5 ) as well as the following fermentations ( 7,8,9 ) are performed in separate containers with specific measurement and control of the temperature, of the mixing degree, of the pH value and of the pressure.

[0001] The invention relates to a method and to an apparatus for non-aerobic digestion of biomasses of different origin, consistency and chemical properties for the production of biogas.

[0002] Methods for the production of biogas with a fermenting machine or reactor on known, wherein complex organic materials (lipids, protids, glucids), which materials are contained in plants or in animal residual products, are destroyed and thereby enable to obtain energy by way of chemical reaction with the aid of enzymes, fungi and microorganisms, wherein these are formed in the biomass in the substrate of a biological origin (organic mass) under in each case defined process conditions. The fermentation is performed thanks to a non-aerobic flora of bacteria, which flora of bacteria is formed depending on the temperature prevailing in the fermentation machine. These bacteria are contained in the biomass and strongly multiply in a closed environment, wherein the enzymes serve as a catalyst; the enzymes react with the organic materials mostly to CH₄ (methane) and CO₂ (carbon dioxide). This course of digestion is performed essentially in three phases:

[0003] hydrolysis of the cellulose, the sugars and the amino acids;

[0004] acidic phase for forming of simple organic acids (for example acetic acid) and alcohols (for example ethyl alcohol);

[0005] methane formation by way of methanogenic bacteria through reaction of acids and alcohols into methane and carbon dioxide.

[0006] The known methods and the plants for their realization to not take sufficiently into consideration that the non-aerobic flora of bacteria is formed of strains of bacteria, which strains of bacteria develop optimally at in each case specific temperatures between 25 degrees centigrade and 45 degrees centigrade and under specific environmental conditions. In addition there exists the risk that a too pronounced acidification occurs during the acidification phase caused by the formation of free acids and therefore the control of the pH value becomes difficult. This causes that the times, within which the individual process steps run, are becoming increased or, respectively, that the residues of the fermentation process are obtained which are not suitable for an advantageous employment of the process, since the residues contain not reacted or only partially reacted components. A too strong acidification can also cause the interruption of the biological course and therewith of the fermentation process. The precedingly recited methods and the corresponding apparatuses do not sufficiently take into consideration the influence of the temperature on the reaction and more generally those of the environmental situations under which the degradation reactions occur. In particular, the conditions for too high an acidification can occur during the phase of the acid formation. The too high formation of free acids and therewith difficult control of the pH values effects a slowing down of the course of the reaction and an incomplete conversion of the substrate with successive maintaining of residual products which are not suitable for later use because of the high content in organic materials.

[0007] It is an object of the present invention to realize a suitable method for the anaerobic digestion of biomasses of the above recited kind as well as a suitable apparatus for this purpose, by way of which is enabled the conversion of the biomass, often the substrate of a biological origin (organic mass) for the purpose of processing biological waste of different origin, different particle size and consistency with different carbon to nitrogen C/N ratios and different humidity, with simultaneous optimation of the duration of the fermentation or, respectively, digestion process, with simultaneous increase of the degree of conversion of the methanogenous substrate, in order to obtain in its manner solid and liquid residues, which solid and liquid residues can be employed as a component in other production processes or in the same production process (recycling) or in different production cycles or in a recycling process.

[0008] It is a further object of the present invention to obtain the following advantages:

[0009] obtaining a higher yield of biogas,

[0010] obtaining solid and liquid residual volumes, which are suitable for the use as additives in production processes.

[0011] For resolving this object the present invention discloses a method, which is subdivided in the following way:

[0012] hydrolysis of the poly saccharides, of the proteins, of the lipids;

[0013] acidification under formation of simple biological acids;

[0014] single or multi-step methanization;

[0015] in order to obtain thereby an easier control of the pH value by separating the acidification step from the methanogenous step and by creating the in each case ideal conditions for the specific requirements of the individual bacteria strains of the anaerobic flora of bacteria by furnishing of a cascade fermentation.

[0016] The present invention furnishes a method for achieving this, wherein the method is subdivided as follows:

[0017] a specific pre-treatment of the diverse biomasses or of parts of the biomasses involving a mechanical comminution and/or a steam treatment and/or the heating to temperatures which effect a “hygienization” of the biomasses;

[0018] a cascade fermentation in order to create ideal conditions for the in each case specific requirements of the individual strands of the flora of the period in order to obtain in particular a methanization during several stages and separate from the acidification phase;

[0019] the feeding back of a part of the sludge, which sludge is produced in the last stage of the process, into the phases of hydrolysis and/or of fermentation with the purpose of feeding in of enzymes, which enzymes operate as biological catalysts;

[0020] the thickening of the sludge by way of a hydro cyclone through separation of residual liquids.

[0021] The apparatus of the invention comprises:

[0022] a cutting plant for the comminution of the biomasses, which cutting plant is employed in particular for the biomasses with a carbon to nitrogen C/N ratio higher than 30 and a humidity content under 30 percent;

[0023] a fermentation machine which comprises separate and different reactors for the purpose of a more effective control of the reaction conditions of the individual process phases;

[0024] a heat treatment with the introduction of hot water vapor directly into the stream of the biomass and/or with a heat exchanger in order to obtain the “hygienization” of certain biomasses (for example starch containing biomasses);

[0025] a heat exchanger for bringing the temperature of the biomass at the discharge after the hydrolysation process to the ideal value for the following acidification process;

[0026] the obtained thermal energy can be used for pre-heating or for feeding thermal energy in one or in several process phases.

[0027] The different devices of the apparatus plant are equipped with thermal sensors and mixers in order to obtain the optimal thermal conditions for the in each case optimal effect of the specific strains of bacteria.

[0028] In the invention is illustrated by way of the accompanying drawings. The accompanying drawings represent schematically the course of the process according to the present invention or, respectively, the apparatus plant according to the present invention for an anaerobic digestion of biomasses of different origin, consistency, and chemical properties, for the generation of biogas, thinking sludge, and liquid residual materials.

[0029]FIG. 1 shows a block diagram of the invention method.

[0030]FIG. 2 shows a block diagram of the apparatus and according to the present invention, wherein the apparatus plant operates according to the method presented in FIG. 1.

[0031] The mixture of 1,2 is taken from storage facilities or silos 1 a, 2 a, which contain bio waste 1 of the type of a large or coarse particle size with a carbon to nitrogen C/N ratio larger than about 30 and a humidity content under 30 percent and organic sludges 2 and/or biomasses of fine particle size with a carbon to nitrogen C/N ratio advantageously below 30 and a content in humidity of advantageously above 30 percent. This charging volume is fed to the cutting plant 3 for the purpose of comminution into smaller particle sizes and is then fed to a preheater by way of a pump 3 a, where the biomass is subjected to a heat addition (Ta—Th) with the heat exchanger 4 a. If required, water vapor V can employed for this purpose, wherein the water vapor can be injected in the charging region, in the discharging region of the heat exchanger into the biomass or into the heat exchanger itself.

[0032] The above recited thermal treatments can be required for the hygienization of the biomass and/or for the thermal splitting of the starches. Usually the pre-heating in the biomass effects a temperature which doesn't surpass 100 degrees centigrade. Thereupon the biomass is fed to the hydrolyzer, which hydrolyzer as well as the following fermentation machines 7,8,9, are equipped with a vertically standing double walled tube 5 a, wherein a cooling agent (Th—Ta) or, respectively, a thermal medium circulates inside the double walled tube 5 a for controlling the temperature.

[0033] The hydrolysis according to the present invention takes place at temperatures between 40 degrees centigrade and 100 degrees centigrade in order to split the organic polymer substrates such as complex starches, pectin, hemicelluloses, under the effect of enzymes (amylases, pectinases, hemicellulases), whereby predominantly mono saccharides and a substrate of low viscosity are formed, which substrate is thereby better available for the successive activity of the methanogenus bacteria. The hydrolysis stage is generally (in known plants) furnished for the generation of biogas, however the generation of biogas occurs at substantially lower temperatures and without control of the temperature and of the intermixing depending on the properties and the concentration of biomass. In contrast, according to the present invention enzymes (amylases, pectinases, hemicellulases) or other materials such as for example several ferments can be added during the hydrolysis of the biomass, whereby eventually also a partially controlled feedback Fe of the sludges F is furnished, wherein the sludges F are taken out at the end of the course of the process.

[0034] It is suitable to cool the substrate in a heat exchanger prior to feeding the substrate into the fermenting machines 7,8,9, wherein possibly the heat transferred to the cooling medium Th—Ta then be employed for the warming or pre-warming of the devices or of parts of the same plant or of a plant connected thereto.

[0035] The fermentation is performed like a cascade in separate fermenting machines 7,8,9 according to the present invention, wherein the separate fermenting machines 7,8,9 are in each case individually controllable and wherein specific cultures that directedly operate under optimum conditions relative to temperature, to intermixing, to the discharge of gas and to the pH value.

[0036] It is known that the different anaerobic strains of bacteria develop their optimum activity in each case at different temperatures: the psicrophilous bacteria at temperatures below 15 degrees centigrade, the mesophilous bacteria at temperatures between 25 degrees centigrade and 45 degrees centigrade, and the thermophilous at temperatures above 45 degrees centigrade.

[0037] The formation of acids and/or the acidification by operation of the acid bacteria is performed in the first fermenting machine 7 always under specific surveillance and monitoring of the temperature, of the pH value, of the intermixing and of the gas discharge. The main culture strains are of the type lactobacillus (for example. lactobacillus casei, lactobacillus plantarum) and of the type streptococcus (for example streptococcus lactii, streptococcus cemoris).

[0038] The proper methanogenus bacteria operate in the following fermenting machines 8,9, wherein the methanogenus bacteria require the controlled feeding of bio genous acids in the substrate coming from the fermenting machine 7. Mixed cultures, predominantly of the type the methanobacterium, methanosarcina, and methanococcus, operate in these fermenting machines. Cellulitis bacteria operate in symbiosis with the methanogenus bacteria, wherein the cellulitis bacteria effect the hydrolysis and the digestion of the organic materials with low solubility, such as for example cellulose.

[0039] Since these anaerobic bacteria multiply only very slowly, and since an anaerobic metabolism exhibits a small energy density, it is necessary to furnish a partial feedback Fe of the sludge F generated at the end of the process into the first phases of the fermentation 5,7.

[0040] The methanogenus bacteria are present in nearly all materials, which are subject to an anaerobic disintegration (sewage sludge, moor sludge, swamp sludge, mining sludge) and are also present in the stomach of the ruminants in symbiosis, and for this reason the excrements of the ruminants are suitable as easily available starting cultures.

[0041] The biogas B is withdrawn from each fermenting machine in a controlled fashion, which biogas B is possibly available as a renewable energy carrier. The completely disintegrated substrate is led from the last fermenting machine 9 into a hydro cyclone 10, wherein the separation of the residual liquid A is performed in the hydro cyclone 10. These components are usable in different production cycles and in fact, for example as a porosity providing agent as it concerns the solid part, or as process water as it concerns the liquid part.

[0042] The invention method and the corresponding apparatus plant enable the processing of biomass of very different origin, consistency and chemical properties, wherein the problems of discharging of liquid residual materials and of thickened sludges are resolved by the complete disintegration of the organic material and wherein the biogas generated, as well as the obtained heat are made useful for the operation of the plant itself or, respectively, as a renewable energy for example for the operation of plants of different kinds.

[0043] Of course, all phases of the method provide for the recording of the data concerning temperature, pressure, flow speed, and chemical composition for the purpose of specific process control, which recording of data allows a high degree of effectiveness concerning the generation of biogas, of sludges F and of residual liquids A, which can be employed without problems in different production processes. 

1. Method for the anaerobic digestion of biomasses with the generation of biogas and sludge, which method includes a hydrolyzation phase and of following fragmentation of the biomass, wherein the method comprises a hydrolyzation phase (5) of the biomass and at least two following fermentation phases (7,8,9) and wherein, be it the hydrolyzation (5) as well as the following fermentations (7,8,9) are performed in separate containers under specific measurement and control of temperature, of intermixing, of the pH value and of the pressure.
 2. Method according to claim 1 wherein the hydrolyzation phase (5) of the biomass is performed at a temperature between 40 degrees centigrade and 100 degrees centigrade, wherein this is followed by a pre-heating phase (4) of the substrate with a heat exchanger (4 a) and/or by way of injection of water vapor (V), and wherein a cooling phase (6) follows to the hydrolyzation phase.
 3. Method according to claim 1 wherein a controlled feeding in is performed of sludges (Fe) containing acid enzymes and generated at the end of the method during the hydrolyzation phase (5) of the biomass and/or in one or in several fermentation phases (7,8,9).
 4. Method according to claim 1 wherein at least one fermentation phase (8,9) is furnished under the subjection with specific methanogenus bacteria after the fermentation (7) which is performed by acid forming enzymes, wherein the specific methanogenus bacteria possibly can be present in the various fermenting machines of different type, wherein specific conditions relating to temperature, pH value, intermixing and pressure prevail in the various fermenting machines.
 5. Method according to claim 1, wherein the sludges (F) occuring at the discharge outlets of the last fermentation machine (9) are thickened in a hydro cyclone (10) or by way of a centrifuge, under separation of the residual liquid (A).
 6. Method according to claim 1 wherein the heat, which is led away during the cooling (6), is employed in one or in several phases (4,5,7,8,9) of the method itself or in another connected process for a pre-heating or heating.
 7. Method according to claim 1 wherein the biogas (B) produced during the different fermentation phases (7,8,9) is withdrawn under controlled conditions from each of the individual fermentation machines.
 8. Method according to claim 1 wherein a hydrolysator (5) is furnished for the anaerobic digestion of the substrate, wherein the hydrolysator (5) is connected in series to at least two fermenting machines (7,8,9) and wherein, be the hydrolysator (5) as well as also the fermenting machines are equipped with heat exchanger (5 a), with mechanical, hydraulic, or gas mixers (5 b) and the sensors or detectors for the determination of the values relating to temperature, the pH value, the pressure, the degree of mixing, and wherein the heat exchanger (5 a), the mixer (5 b), the withdrawal of biogas and the introduction of the fed back sludges (Fe) are operated depending on the recited determined values and of the specific requirements of the flora of bacteria operating in the hydrolysator (5) and in the different fermenting machines (7,8,9).
 9. Apparatus according to claim 8 wherein prior to the hydrolysis (5) there is furnished a preheater (4) with the exchanger (4 a) and a device for the injection (4 b) of water vapor prior to the exchanger (4 a), after the heat exchanger (4 a), or in an intermediate position.
 10. Apparatus according to claim 8 wherein a heat exchanger (6) is furnished after the hydrolysator (5) for cooling of the substrate.
 11. Apparatus according to claim 8 wherein a controlled feedback (Fe) of the sludges, which sludges are separated at the end of the apparatus, is performed by introduction into the hydrolysator (5) and/or into one or into several fermentation machines (7,8,9). 